The present invention relates to a method of scanning in a magnetic resonance imaging (MRI) method, and more particularly to the method in which slice positions to be scanned in an object being diagnosed are decided before a scanning for diagnosis.
In MRI, an object to be diagnosed is placed in a static magnetic field, whereby atomic nuclei align themselves with the static magnetic field. Then, gradient magnetic fields in three X-, Y- and Z-directions are applied to the object for spatially encoding and a radio frequency (RF) signal is applied to the object for exciting the atomic nuclei in a magnetically sliced plane, which has a certain thickness in a slicing direction, of the object. When the RF signal is removed, magnetic resonance (MR) signals emitted from the sliced plane can be collected. A series of the excitation and MR signal acquisition is performed on a predetermined pulse sequence. The collected MR signals are then processed, for example, by Fourier transformation to form image data of the magnetically sliced plane of the object.
Prior to a scanning for diagnosis in accordance with the above MRI principle, it is usual to perform a preparation step which includes a step of designating a slice position in the scanning for diagnosis. For this step a single sliced image is chosen as a pilot image. The term "pilot image" is here given for this positioning step only.
The positioning using the single pilot image further includes two ways. One is to designate slicing planes crossed perpendicularly to the pilot image displayed on a monitor. This way is called here "cross" plan. The other way involves the designation of slicing planes parallel to the pilot image. This way is called here "same" plan.
Now, the positioning according to the above two ways will be exemplified. By the "cross" plan, for example, a sagittal image of the head of a patient as a pilot image will be first displayed on a monitor. While looking at the sagittal image, an operator designates quantitatively not only a desired scanning range and its center for the object being imaged (i.e., head) in a reading direction (or in an encoding (i.e, phase-encoding) direction), but also a desired slicing range in a slicing direction. At the same time, by using the same sagittal image, the operator is forced to give by his or her approximate estimate (i.e., by intuition) a desired scanning range and its center for the object in the encoding direction (or in the reading direction) whose directional information is not appeared on the monitor screen. According to the designated figures, the scanning will be performed to produce axial images for diagnosis, perpendicular to the sagittal image as the pilot image, of the desired slicing number. The same manner as above can be applied to a patient's abdomen scanning. For instance, the coronal image of the abdomen as a pilot image can be used to produce axial images for diagnosis.
The scanning range and its center thus-designated correspond to a field of view and its center, when diagnostic images are displayed.
However, the "cross plan" has various drawbacks. As said above, it is possible for an operator to quantitatively designate an object center position and a desired scanning range in either of the reading or encoding direction on a pilot image, but it is impossible to designate with confidence those figures in the remaining direction, because information regarding the remaining direction does not appear on the monitor.
Thus, such dependence on an operator's intuition in the designation sometimes causes unfavorable conditions. The object center tends to deviate from the center position of a field of view (FOV) of a monitor screen. This deviation causes a part of the object to go out of the FOV, resulting in the generation of folded artifacts. Hence, it is not easy to utilize a technique being able to changing image ranges, such as a rectangular FOV method and an arbitrary matrix method for reduction in scanning time. There is no guarantee that the operator's intuition with experience is always right. If a FOV designated by the operator's intuition is too small for an object being imaged, folded artifacts will be appeared on axial images obtained in the diagnostic scanning. Contrary, where a rather bigger FOV than an object field range is designated for safety, folded artifacts will be expelled as expected, but longer scanning time is required instead. As a result, it is meaningless to adopt FOV-adjustable techniques.
On the other hand, the above-mentioned "same" plan will be carried out as follows. Suppose that an object being imaged is a patient's head. In this case, for example, an axial image as a single pilot image will be displayed on a monitor. With an operator looking at the monitor screen, he or she is required to quantitatively designate desired scanning ranges and their centers in both the reading and encoding directions. Also required is to give a necessary slicing ranges by his or her intuition with experience. The designation enables the scanning carried out in succession to produce a plurality of parallel axial images for diagnoses.
The "same" plan discussed above has no problems with respect to quantitative designation of the scanning ranges and their centers in both the encoding and reading directions. However, it has no way for confirming a practically-sliced range prior to the scanning. This means that the designation on an operator's intuition sometimes leads to an improper slicing number, a slicing width, and/or slicing gap. This will fail to exactly catch a region being imaged by an entire magnetically-sliced region, thereby requiring to carry out another scanning from the beginning